Power supply unit with autonomous input power limiting

ABSTRACT

A power supply receives AC power and generates a DC output voltage. The power supply may be divided into a primary section that converts AC power to a relatively high DC voltage. A secondary section converts this relatively high DC voltage into a well-regulated lower DC voltage. In an embodiment, the current and/or power supplied by the primary to the secondary side is used by the secondary side in a closed-loop feedback system to limit the current drawn by the secondary side to a configurable value.

TECHNICAL FIELD

Aspects of the disclosure are related to the field of power supplyunits, and in particular, to optimizing system level input powermanagement by providing a power supply unit with highly accurate andautonomous input power limiting capabilities.

BACKGROUND

This section provides background information related to the presentdisclosure which is not necessarily prior art.

A power supply unit is an electrical device that supplies electric powerto an electrical load. Indeed, power supply units typically have a powerinput connection, which receives energy in the form of electric currentfrom a source, and one or more power output connections that delivercurrent to the load. The primary function of a power supply is toconvert electric current from a source to a correct voltage, current,and frequency to power a load. Indeed, a power supply unit may perform avariety of functions, such as, but not limited to, power conversion,alternating current to direct current (AC-DC) or DC-DC conversion,adjusting voltage levels, and providing backup power during power gridoutages.

A power supply system typically includes multiple power sources (orpower supply units) that provide power and an external system (orcontroller) that provides power management functionality including inputcurrent limiting for the multiple power sources. Power limiting is thepractice in electrical or electronic circuits of imposing an upper limiton the power (e.g., current) that may be delivered to a load with thegeneral purpose of protecting the circuit generating or transmitting thecurrent from harmful effects due to a short-circuit or similar problemin the load. In the context of data centers, input power limiting istypically utilized to control power distribution and provide loadsharing amongst the multiple power sources. For example, an externalsystem (or external controller) typically handles power management byrelying on an alert signal asserted by a power supply unit to determinewhen a load is reaching an over current warning level. Indeed, when apower supply unit reaches a threshold current (e.g., over currentwarning level), the power supply unit asserts the alert signal toappropriately inform the external system (or controller). The externalsystem (or controller) responsively reduces the processing utilizationassociated with the power supply unit to lower the load which, in turn,reduces the input the input power (e.g., current) drawn by the powersupply unit.

Unfortunately, conventional power supply systems suffer numerousdeficiencies with respect to input power limiting. For example, responsetimes for adjusting the input power are often slow and inaccuratebecause adjustments to the input power must be made by the externalsystem (or controller). Furthermore, the external system (or controller)must utilize extensive processing resources to compute equivalent outputpower limiting for the power supply units. That is, the external system(or controller) must determine how to modify the load associated witheach power supply unit by adjusting the corresponding processingutilization.

SUMMARY

One or more embodiments described herein, among other benefits, solveone or more of the foregoing or other problems in the art by providingsystems, methods, and non-transitory computer readable media forlimiting input current and/or power drawn by a power supply unit. Insome implementations, the technology described includes a methodcomprising: generating an output current adjustment based on an inputpower and an input power limit; and, regulating an output voltage basedon an output current and the output current adjustment.

In some implementations, the technology described includes a method ofregulating a secondary side output current, comprising: regulating anoutput voltage using closed-loop current feedback; generating an erroradjustment signal based on a primary side input power and a target powerlimit; and, using the error adjustment signal to limit an output currentof the output voltage.

In some implementations, the technology described includes a powersupply system, comprising: a secondary output voltage regulator thatuses a closed-loop current feedback signal to produce an output voltage;an error adjustment signal that is based on a primary side input powerand a target input power limit; and, a current feedback combiner to,based on the error adjustment signal, change the closed-loop referencesignal to limit an output current.

Additional features and advantages of the present application will beset forth in the description which follows, and in part will be obviousfrom the description, or may be learned by the practice of such exampleembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to describe the manner in which the above-recited and otheradvantages and features can be obtained, a more particular descriptionis set forth and will be rendered by reference to specific examplesthereof which are illustrated in the appended drawings. Understandingthat these drawings depict only typical examples and are not consideredto be limiting of its scope. Implementations will be described andexplained with additional specificity and detail through the use of theaccompanying drawings.

FIG. 1 is a block diagram illustrating a power supply system.

FIG. 2 is a block diagram illustrating a closed-loop input current/powerlimited power supply system.

FIG. 3 is a flowchart illustrating a method of changing a power supplypower limit.

FIG. 4 is a block diagram illustrating a system.

FIG. 5 is a flowchart illustrating a method of operating a power supply.

FIG. 6 is a flowchart illustrating a method of regulating a secondaryside output current.

FIG. 7 is a block diagram illustrating a computer system.

The drawings have not necessarily been drawn to scale. Similarly, somecomponents and/or operations may be separated into different blocks orcombined into a single block for the purposes of discussion of some ofthe embodiments of the present technology. Moreover, while thetechnology is amenable to various modifications and alternative forms,specific embodiments have been shown by way of example in the drawingsand are described in detail below. The intention, however, is not tolimit the technology to the particular embodiments described. On thecontrary, the technology is intended to cover all modifications,equivalents, and alternatives falling within the scope of the technologyas defined by the appended claims.

DETAILED DESCRIPTION

Example implementations are provided so that this disclosure will bethorough, and will fully convey the scope to persons skilled in the art.Numerous specific details are set forth such as examples of specificcomponents, devices, and methods, to provide a thorough understanding ofimplementations of the present disclosure. It will be apparent to thoseskilled in the art that specific details need not be employed, thatexample implementations may be embodied in many different forms and thatneither should be construed to limit the scope of the disclosure. Insome example implementations, well-known processes, well-known devicestructures, and well-known technologies are not described in detail.

The terminology used herein is for the purpose of describing particularexample implementations only and is not intended to be limiting. As usedherein, the singular forms “a,” “an,” and “the” may be intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. The terms “comprises,” “comprising,” “including,” and“having,” are inclusive and therefore specify the presence of statedfeatures, integers, steps, operations, elements, and/or components, butdo not preclude the presence or addition of one or more other features,integers, steps, operations, elements, components, and/or groupsthereof. The method steps, processes, and operations described hereinare not to be construed as necessarily requiring their performance inthe particular order discussed or illustrated, unless specificallyidentified as an order of performance. It is also to be understood thatadditional or alternative steps may be employed.

Although the terms first, second, third, etc., may be used herein todescribe various elements, components, regions, layers and/or sections,these elements, components, regions, layers and/or sections should notbe limited by these terms. These terms may be only used to distinguishone element, component, region, layer or section from another region,layer or section. Terms such as “first,” “second,” and other numericalterms when used herein do not imply a sequence or order unless clearlyindicated by the context. Thus, a first element, component, region,layer, or section discussed below could be termed a second element,component, region, layer, or section without departing from theteachings of the example embodiments.

Spatially relative terms, such as “inner,” “outer,” “beneath,” “below,”“lower,” “above,” “upper,” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. Spatiallyrelative terms may be intended to encompass different orientations ofthe device in use or operation in addition to the orientation depictedin the figures. For example, if the device in the figures is turnedover, elements described as “below” or “beneath” other elements orfeatures would then be oriented “above” the other elements or features.Thus, the example term “below” can encompass both an orientation ofabove and below. The device may be otherwise oriented (rotated 90degrees or at other orientations) and the spatially relative descriptorsused herein interpreted accordingly.

In an embodiment, a power supply receives AC power and generates a DCoutput voltage. The power supply may be divided into a primary sectionthat converts AC power to a relatively high DC voltage. A secondarysection converts this relatively high DC voltage into a well regulatedlower DC voltage. In an embodiment, the current and/or power supplied bythe primary to the secondary side is used by the secondary side in aclosed-loop feedback system to limit the current drawn by the secondaryside to a configurable value.

FIG. 1 is a block diagram illustrating a power supply system. In FIG. 1, system 100 comprises external system controller 110, power supply unit120, and load 150. Power supply unit 120 includes closed-loop inputpower (CLIP) circuitry 122. CLIP 122 includes power limit storage 123.External system controller 110 is operatively coupled to PSU 120.External system controller 110 is operatively coupled to PSU 120 toprovide PSU 120 with at least a input power limit 115 that is stored bypower limit storage 123. PSU 120 is operatively coupled to load 150. PSU120 is operatively coupled to load 150 to provide load 150 with avariable amount of output current 125 at a regulated DC voltage. PSU 120also receives a varying indicator of measured input power 116.

It should be understood that for the purposes of this disclosure,because power supplies are supplied with a constant input voltage, butdraw a varying amounts of input current depending upon load, inputcurrent and input power can be used interchangeably to refer to inputpower and/or input current. For the same reasons, input current limitand input power limit can be used interchangeably to refer to inputpower limits and/or input current limits. Likewise, because powersupplies provide loads with a constant output voltage, but supply avarying amount of output current to that load, output current and outputpower can be used interchangeably to refer to output power and/or outputcurrent. input current limit and input power limit can be usedinterchangeably to refer to input power limits and/or input currentlimits.

In an embodiment, the amount of output current 125 supplied to load 150affects the amount of input current drawn by PSU 120. Because the inputcurrent drawn by PSU 120 is indicated by measured input power 116, CLIPcan rely on a feedback loop to limit the input power to PSU 120. Thisfeedback loop includes a comparison of measured input power 116 to theinput power limit 115 that is stored by power limit storage 123, and theoutput current 125. In other words, based on the input power limit 115stored by power limit storage 123 and the measured input power 116, CLIPadjusts the amount of current supplied to load 150. This in-turn affectsthe measured input power completing the feedback loop. CLIP adjusts theamount of current supplied to load 150 to limit the measured input power116 to at or below the amount of current indicated by power limitstorage 123.

FIG. 2 is a block diagram illustrating a closed-loop input current/powerlimited power supply system. In FIG. 2 , system 200 comprises powercontroller 210, power supply unit 220, and AC power source 228. Powersupply unit 220 includes primary side circuitry 221 and secondary sidecircuitry 230. Secondary side circuitry 230 includes compare (subtract)231, error indicator 232, proportional-integral-derivative (PID) 233,adjustment indicator 234, reference current 235, scaled currentreference indicator 236, scaler 237, and regulator 240. Regulator 240includes compare (subtract) 245, error indicator 246, pulse-widthmodulation (PWM) controller 241, plant 242, regulated output 243, andoutput current indicator 244.

AC power source 228 supplies varying power (varying current at a fixedvoltage) to primary side circuitry 221 according to the needs of PSU220. Primary side circuitry 221 supplies varying current 222 (again,varying current at a fixed voltage) to plant 242. Primary side circuitry221 also provides a varying input current indicator 225 to compare 231.Varying input current indicator 225 conveys information about the amountof current being supplied by power source 228 to primary side circuitry221.

Compare 231 also receives input current limit 215. Compare 231 subtractsinput current indicator 225 from current limit indicator 215 to generateerror indicator 232. Thus, error indicator 232 is a measure of how closeinput current indicator 225 is to meeting or exceeding current limitindicator 215. PID 233 receives error indicator 232 and outputsadjustment indicator 234.

Adjustment indicator 234 may be viewed as a percentage of full currentcapacity that regulator 240 is allowed to supply. Adjustment indicator234 may vary between indications of from 0% to 100% (as with manyfeedback control systems.) Adjustment indicator 234 indicates 100% aslong as error indicator 232 is positive (i.e., input current indicator225 is less than input current limit 215.) When error indicator 232 isnegative (i.e., input current indicator 225 is greater than inputcurrent limit 215) PID 233 fractionally reduces adjustment indicator 234to decrease the amount of current output by regulator 240.

Scaler 237 effectively multiplies reference current 235 by adjustmentindicator 234 and outputs scaled current reference indicator 236. Thus,scaled current reference indicator 236 effectively sets the maximumcurrent to be output by regulator 240 as a percentage of referencecurrent 235. In other words, when adjustment indicator 234 is indicating100%, the maximum current allowed to be output by regulator 240 isreference current 235. When adjustment indicator 234 is indicating 50%,the maximum current allowed to be output by regulator 240 is 50% ofreference current 235, and so on.

Regulator 240 receives scaled current reference indicator 236. Compare245 receives scaled current reference indicator 236 and an outputcurrent indicator 244 provided by plant 242. Compare 245 subtractsoutput current indicator 244 from scaled current reference indicator 236to generate error indicator 246. Thus, error indicator 246 is a measureof how close output current indicator 244 is to meeting or scaledcurrent reference indicator 236. PWM controller 241 receives errorindicator 246 and controls plant 242 to provide a limited current onregulated output 243 according to error indicator 246.

Thus, it should be understood that compare 245, PWM controller 241, andplant 242 are parts of a secondary side feedback loop that limits thecurrent output by regulated output 243 to a target value. It should alsobe understood that because plant 242 of regulator 240 receives current222 from primary side circuitry 221, regulator 240, primary sidecircuitry 221, compare 231, PID 233, and scaler 237 are part of aprimary side feedback loop that limits input current from power source228.

FIG. 3 is a flowchart illustrating a method of changing a power supplypower limit. One or more steps illustrated in FIG. 3 may be performedby, for example, system 100, system 200, and/or their components. Acommand is received to change an input power limit (302). For example,PSU 120 may receive a command from external system controller 110 tochange the input power limit 115 stored in power limit storage 123.

The actual input power is read (304). For example, CLIP 122 may receivemeasured input power 116. Closed-loop control of the input power isresumed (306). For example, CLIP 122 may resume closed-loop controlusing the new input power limit 115 stored in power limit storage 123and the measured input power 116.

FIG. 4 is a block diagram illustrating a system. coupled to a powersupply. In FIG. 4 , system 400 includes AC power source 428, primaryside circuitry 420, secondary side circuitry 430, and load system 450.Load system 450 include load 451 and system control 410. Primary sidecircuitry 420 include primary controller 421, AC-DC power converter 429,and portion of DC-DC power converter 440. Secondary side circuitry 430includes the remaining portion of DC-DC power converter 440, andsecondary controller 441. Primary side circuitry 420 and secondary sidecircuitry 430 are separated by isolation 425.

It should be understood that primary side circuitry 420 may be, include,or implement all or parts of primary side circuitry 221. Secondary sidecircuitry 430 may be, include, or implement all or parts of secondaryside circuitry 230. Together, primary side circuitry 420 and secondaryside circuitry 430 may be, include, or implement all or parts of CLIP122, PSU 120, and/or PSU 220.

AC-DC power converter 429 receives AC power from AC power source 428.AC-DC power converter 429 converts AC power from AC power source 428 toDC supply voltage V_(bulk). V_(bulk) is provided to DC-DC powerconverter 440. AC-DC power converter 429 is controlled by primarycontroller 421. Primary controller 421 receives information about ACpower source 428 (e.g., input current indicator).

Primary controller 421 is operatively coupled to secondary controller441 and system controller 410. Primary controller 421 is operativelycoupled to system controller 410 to transmit or receive commands and/orother information (e.g., input current limit indicator) used to controlAC-DC power converter 429.

DC-DC power converter 440 receives DC supply voltage V_(bulk). DC-DCpower converter 440 converts DC power from DC supply voltage V_(bulk) toDC supply voltage V_(out). V_(out) is provided to load system 450 andload 451, in particular.

Secondary controller 441 is operatively coupled to primary controller421 and system controller 410. Secondary controller 441 is operativelycoupled to system controller 410 to transmit or receive commands and/orother information (e.g., output current limit indicator) used to controlDC-DC power converter 440. Secondary controller 441 is operativelycoupled to primary controller 421 to transmit or receive commands and/orother information (e.g., input current indicator) used to control DC-DCpower converter 440.

FIG. 5 is a flowchart illustrating a method of changing a power supplypower limit. One or more steps illustrated in FIG. 5 may be performedby, for example, system 100, system 200, system 400, and/or theircomponents. An output current adjustment is generated based on an inputpower and an input power limit (502). For example, scaled currentreference indicator 236 may be generated by compare 231, PID 233, andscaler 237 based on input current limit 215 and input current indicator225.

An output voltage is regulated based on an output current and the outputcurrent adjustment (504). For example, regulator 240 may regulate output243 based on scaled current reference indicator 236 and output currentindicator 244.

FIG. 6 is a flowchart illustrating a method of changing a power supplypower limit. One or more steps illustrated in FIG. 6 may be performedby, for example, system 100, system 200, system 400, and/or theircomponents. An output voltage is regulated using a closed-loop currentfeedback (602). For example, regulator 240 may regulate output 243 basedon a closed feedback loop that includes output current indicator 244.

An error adjustment signal is generated based on a primary side inputcurrent and a target current limit (604). For example, scaled currentreference indicator 236 may be generated by compare 231, PID 233, andscaler 237 based on input current limit 215 and input current indicator225. The error adjustment signal is used to limit an output current fthe output voltage (606). For example, regulator 240 may, based onscaled current reference indicator 236, limit the current provided byoutput 243.

FIG. 7 is a block diagram illustrating a computer system. In anembodiment, computer system 700 and/or its components include circuits,software, and/or data that implement, or are used to implement, themethods, systems and/or devices illustrated in the Figures, thecorresponding discussions of the Figures, and/or are otherwise taughtherein. The methods described above may also be stored on anon-transitory computer readable medium. Devices, circuits, and systemsdescribed herein may be implemented using computer-aided design toolsavailable in the art, and embodied by computer-readable files containingsoftware descriptions of such circuits. This includes, but is notlimited to one or more elements of system 100, system 200, system 400,and/or their components. These software descriptions may be: behavioral,register transfer, logic component, transistor, and layoutgeometry-level descriptions.

Data formats in which such descriptions may be implemented are stored ona non-transitory computer readable medium include, but are not limitedto: formats supporting behavioral languages like C, formats supportingregister transfer level (RTL) languages like Verilog and VHDL, formatssupporting geometry description languages (such as GDSII, GDSIII, GDSIV,CIF, and MEBES), and other suitable formats and languages. Physicalfiles may be implemented on non-transitory machine-readable media suchas: 4 mm magnetic tape, 8 mm magnetic tape, 3½-inch floppy media, CDs,DVDs, hard disk drives, solid-state disk drives, solid-state memory,flash drives, and so on.

Alternatively, or in addition, the functionally described herein can beperformed, at least in part, by one or more hardware logic components.For example, and without limitation, illustrative types of hardwarelogic components that can be used include Field-programmable Gate Arrays(FPGAs), Application-specific Integrated Circuits (ASICs),Application-specific Standard Products (ASSPs), System-on-a-chip systems(SOCs), Complex Programmable Logic Devices (CPLDs), multi-coreprocessors, graphics processing units (GPUs), etc.

FIG. 7 illustrates a block diagram of an example computer system.Computer system 700 includes communication interface 720, processingsystem 730, storage system 740, and user interface 760. Processingsystem 730 is operatively coupled to storage system 740. Storage system740 stores software 750 and data 770. Processing system 730 isoperatively coupled to communication interface 720 and user interface760. Computer system 700 may comprise a programmed general-purposecomputer. Computer system 700 may include a microprocessor. Computersystem 700 may comprise programmable or special purpose circuitry.Computer system 700 may be distributed among multiple devices,processors, storage, and/or interfaces that together comprise elements720-770.

Communication interface 720 may comprise a network interface, modem,port, bus, link, transceiver, or other communication device.Communication interface 720 may be distributed among multiplecommunication devices. Processing system 730 may comprise amicroprocessor, microcontroller, logic circuit, or other processingdevice. Processing system 730 may be distributed among multipleprocessing devices. User interface 760 may comprise a keyboard, mouse,voice recognition interface, microphone and speakers, graphical display,touch screen, or other type of user interface device. User interface 760may be distributed among multiple interface devices. Storage system 740may comprise a disk, tape, integrated circuit, RAM, ROM, EEPROM, flashmemory, network storage, server, or other memory function. Storagesystem 740 may include computer readable medium. Storage system 740 maybe distributed among multiple memory devices.

Processing system 730 retrieves and executes software 750 from storagesystem 740. Processing system 730 may retrieve and store data 770.Processing system 730 may also retrieve and store data via communicationinterface 720. Processing system 750 may create or modify software 750or data 770 to achieve a tangible result. Processing system may controlcommunication interface 720 or user interface 760 to achieve a tangibleresult. Processing system 730 may retrieve and execute remotely storedsoftware via communication interface 720.

Software 750 and remotely stored software may comprise an operatingsystem, utilities, drivers, networking software, and other softwaretypically executed by a computer system. Software 750 may comprise anapplication program, applet, firmware, or other form of machine-readableprocessing instructions typically executed by a computer system. Whenexecuted by processing system 730, software 750 or remotely storedsoftware may direct computer system 700 to operate as described herein.

Implementations discussed herein include, but are not limited to, thefollowing examples:

Example 1

A method, comprising: generating an output current adjustment based onan input power and an input power limit; and, regulating an outputvoltage based on an output current and the output current adjustment.

Example 2

The method of claim 1, wherein the output current adjustment is furtherbased on a constant current reference.

Example 3

The method of claim 2, wherein the output current adjustment is based ona difference between the input power and the input power limit.

Example 4

The method of claim 3, wherein the output current adjustment decreasesthe output current when the input power exceeds the input power limit.

Example 5

The method of claim 4, wherein the output current adjustment does notaffect the output current when the input power is less than the inputpower limit.

Example 6

The method of claim 5, wherein the input power limit is configurable.

Example 7

The method of claim 6, wherein the input power is received from aprimary side power supply.

Example 8

A method of regulating a secondary side output current, comprising:regulating an output voltage using closed-loop current feedback;generating an error adjustment signal based on a primary side inputcurrent and a target current limit; and, using the error adjustmentsignal to limit an output current of the output voltage.

Example 9

The method of claim 8, wherein the error adjustment signal is furtherbased on a constant current reference.

Example 10

The method of claim 9, wherein the error adjustment signal is based on adifference between the primary side input power and the target currentlimit.

Example 11

The method of claim 10, wherein the error adjustment signal causes theclosed-loop current feedback to decrease the output current when theprimary side input power exceeds the target current limit.

Example 12

The method of claim 11, wherein the error adjustment signal does notaffect the output current when the primary side input power is less thanthe target current limit.

Example 13

The method of claim 12, wherein the target current limit isconfigurable.

Example 14

The method of claim 13, wherein the closed-loop current feedbackincludes a pulse-width modulation controller.

Example 15

A power supply system, comprising: a secondary output voltage regulatorthat uses a closed-loop current feedback signal to produce an outputvoltage; an error adjustment signal that is based on a primary sideinput current and a target current limit; and, a current feedbackcombiner to, based on the error adjustment signal, change theclosed-loop feedback signal to limit an output current.

Example 16

The power supply system of claim 15, wherein the secondary outputvoltage regulator includes a pulse-width modulation controller.

Example 17

The power supply system of claim 16, further comprising: a constantcurrent reference.

Example 18

The power supply system of claim 17, further comprising: a constantcurrent reference modulator to, based on the primary side input currentand the target current limit, scale the constant current reference toproduce the error adjustment signal.

Example 19

The power supply system of claim 18, wherein the target current limit isconfigurable by an external system.

Example 20

The power supply system of claim 18, further comprising:proportional-integral-derivative feedback to scale the constant currentreference to produce the error adjustment signal.

As will be appreciated by one skilled in the art, aspects of the presentinvention may be embodied as a system, method, or computer programproduct. Accordingly, aspects of the present invention may take the formof an entirely hardware embodiment, an entirely software embodiment(including firmware, resident software, micro-code, etc.) or anembodiment combining software and hardware aspects that may allgenerally be referred to herein as a “circuit,” “module” or “system.”Furthermore, aspects of the present invention may take the form of acomputer program product embodied in one or more computer readablemedium(s) having computer readable program code embodied thereon.

The included descriptions and figures depict specific embodiments toteach those skilled in the art how to make and use the best mode. Forthe purpose of teaching inventive principles, some conventional aspectshave been simplified or omitted. Those skilled in the art willappreciate variations from these embodiments that fall within the scopeof the disclosure. Those skilled in the art will also appreciate thatthe features described above may be combined in various ways to formmultiple embodiments. As a result, the invention is not limited to thespecific embodiments described above, but only by the claims and theirequivalents.

What is claimed is:
 1. A method, comprising: generating constant currentadjustment based on an input power and an input power limit; scaling areference current based on the constant current adjustment; generatingan output current adjustment based on the scaled reference current; andregulating an output voltage based on an output current and the outputcurrent adjustment.
 2. The method of claim 1, wherein the referencecurrent comprises a constant current reference.
 3. The method of claim2, wherein the output current adjustment is based on a differencebetween the input power and the input power limit.
 4. The method ofclaim 3, wherein the output current adjustment decreases the outputcurrent when the input power exceeds the input power limit.
 5. Themethod of claim 4, wherein the output current adjustment does not affectthe output current when the input power is less than the input powerlimit.
 6. The method of claim 5, wherein the input power limit isconfigurable.
 7. The method of claim 6, wherein the input power isreceived from a primary side power supply.
 8. A method of regulating asecondary side output current, comprising: regulating an output voltageusing closed-loop current feedback; generating a current adjustmentsignal based on a primary side input power and a target power limit;scaling a reference current based on the current adjustment signal;generating an error adjustment signal based on the scaled referencecurrent; and using the error adjustment signal to limit an outputcurrent of the output voltage.
 9. The method of claim 8, wherein thereference current comprises a constant current reference.
 10. The methodof claim 9, wherein the error adjustment signal is based on a differencebetween the primary side input power and the target power limit.
 11. Themethod of claim 10, wherein the error adjustment signal causes theclosed-loop current feedback to decrease the output current when theprimary side input power exceeds the target power limit.
 12. The methodof claim 11, wherein the error adjustment signal does not affect theoutput current when the primary side input power is less than the targetpower limit.
 13. The method of claim 12, wherein the target power limitis configurable.
 14. The method of claim 13, wherein the closed-loopcurrent feedback includes a pulse-width modulation controller.
 15. Apower supply system, comprising: a secondary output voltage regulatorthat uses a closed-loop current feedback signal to produce an outputvoltage; an error adjustment signal that is based on a primary sideinput power and a target power limit; a current feedback combiner to,based on the error adjustment signal, change the closed-loop feedbacksignal to limit an output current; a constant current reference; and aconstant current reference modulator to, based on the primary side inputpower and the target power limit, scale the constant current referenceto produce the error adjustment signal.
 16. The power supply system ofclaim 15, wherein the secondary output voltage regulator includes apulse-width modulation controller.
 17. The power supply system of claim15, wherein the target power limit is configurable by an externalsystem.
 18. The power supply system of claim 15, further comprising:close-loop feedback control to scale the constant current reference toproduce the error adjustment signal.